build: fix node_g target

Currently when running make node_g the following error is displayed:
if [ ! -r node -o ! -L  ]; then ln -fs out/Debug/node node_g; fi
/bin/sh: line 0: [: argument expected

It looks like there was a typo for the NODE_EXE where node became
lowercase instead of uppercase.

Ref: https://github.com/nodejs/node/pull/9827
PR-URL: https://github.com/nodejs/node/pull/10153
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>

Makefile

@@ -71,11 +71,11 @@ endif
 # See comments on the build-addons target for some more info
 $(NODE_EXE): config.gypi out/Makefile
 	$(MAKE) -C out BUILDTYPE=Release V=$(V)
-	if [ ! -r $(NODE_EXE) -o ! -L $(NODE_EXE) ]; then ln -fs out/Release/$(NODE_EXE) $@; fi
+	if [ ! -r $@ -o ! -L $@ ]; then ln -fs out/Release/$(NODE_EXE) $@; fi
 
 $(NODE_G_EXE): config.gypi out/Makefile
 	$(MAKE) -C out BUILDTYPE=Debug V=$(V)
-	if [ ! -r $(NODE_EXE) -o ! -L $(node_EXE) ]; then ln -fs out/Debug/$(NODE_EXE) $@; fi
+	if [ ! -r $@ -o ! -L $@ ]; then ln -fs out/Debug/$(NODE_EXE) $@; fi
 
 out/Makefile: common.gypi deps/uv/uv.gyp deps/http_parser/http_parser.gyp \
               deps/zlib/zlib.gyp deps/v8/gypfiles/toolchain.gypi \